In a frequency hopping (or channel hopping) mesh network, devices communicate using different frequencies/channels at different times. To communicate a packet, a transmitter-receiver pair must be configured to the same channel during packet transmission. For a transmitter to communicate with a receiver at an arbitrary time in the future, the transmitter and receiver must synchronize to a channel schedule that specifies to what channel to communicate on at what time. That is, to communicate a message, the transmitter must transmit the message according to the receiver's frequency hopping schedule. Note that the transmitter will often need to transmit the message on a channel different from its own receiving schedule.
Many communication devices, particularly in low-power and lossy network (LLNs), typically communicate using only a single transceiver due to cost or energy constraints. Most narrow band transceivers in use today can only be configured to transmit or receive on a single channel at a time. That is, while configured for a particular channel, the transceiver cannot transmit or receive on any other channel, and since transceivers are typically half-duplex, they do not have the ability to transmit and receive at the same time.
Because a narrow-band transceiver is limited to a single channel and is half-duplex, determining the cause of a packet drop effectively in a carrier sense multiple access (CSMA)-based network can be challenging. In particular, when a device “A” fails to receive an acknowledgment from a device “B,” it does not know if the transmission was lost due to link quality issues or simply because device B was tuned to a different channel and/or transmitting at the same time. Also, though CSMA-based systems typically employ random backoffs and clear-channel assessment mechanism to help avoid collisions, in a frequency hopping system, the traditional clear-channel assessment mechanism no longer applies.